1. Field of the Invention
The disclosures herein relate to an image forming apparatus and a method for forming an image.
2. Description of the Related Art
In a typical image forming apparatus, images are optically read via an image reader such as a scanner; however, when the images are being read, some of the image signals may be lost or some of the image signals read by the image reader may exhibit inaccurate values due to defects in an image sensor that reads image signals or due to presence of contaminated marks in an optical path such as contaminated marks on a contact glass. Further, since the image reader is composed of plural tiny contact image sensors, the image reader may fail to acquire image signals in gaps between the contact image sensors. Accordingly, the image read by the image reader in such conditions may be degraded due to lack of pixel values or presence of inaccurate pixels.
In order to overcome such image degradation, there are ordinary interpolation methods known to the art, one example of which includes estimating accurate pixel values corresponding to those of missing pixels or of inaccurate pixels by polynomial interpolation or pattern matching utilizing pixels peripheral to the missing or inaccurate pixels, and replacing the inaccurate pixel values derived from lack of pixels or presence of inaccurate pixels with the estimated pixel values.
With such an ordinary interpolation method, pixel values to be interpolated may be accurately estimated in an image having gradually changed pixel values; however, the accuracy of the estimated pixel values may be significantly degraded in an image having drastically changed pixel values. An example of the former (image with gradually changed pixel values) may be a photograph. In the image having gradually changed pixel values, since a sampling cycle for sampling an image is sufficiently shorter than a fluctuating cycle of the image pattern, pixel values to be interpolated may be accurately estimated by general polynomial interpolation. An example of the latter (image with drastically changed pixel values) may be a halftone image. In the image having drastically changed pixel values, resolution of the image needs to be sufficiently high comparative to lines per inch of the halftone image. If the resolution of the image is not sufficiently high, the sampling cycle may not be sufficiently short relative to the fluctuation cycle of the image pattern, and the original image pattern may not be reproduced by the general polynomial interpolation technique as a result. For example, if the image reader composed of contact image sensors fails to read a row of pixels due to gaps between the contact image sensors while reading the halftone image, and pixels corresponding to the missing row of pixels are interpolated by utilizing the ordinary interpolation, interpolation errors may periodically occur. The interpolation errors may result in pin stripes in the pixel interpolated image and hence, the pixel interpolated image may be significantly degraded. If pixels corresponding to the missing row of pixels are interpolated by utilizing the pattern matching, an appropriate analogous pattern may not be found near the pixels subject to interpolation due to a relationship between the resolution of the image and the lines per inch or a screen angle. If the search range (area) of the analogous pattern is increased, the pixel interpolation may be easily affected by changes in contents-dependent colors or luminance of the image.
In the ordinary interpolation method, there may not be a sufficient number of pixels utilized for interpolating the changed pixel values. Even if the pattern has a short cycle but the pattern is sampled by a sufficient sampling frequency, the original pattern may be accurately reproduced. Therefore, in order to estimate the pixel values of interpolating pixels (i.e., pixels utilized for interpolation) with high accuracy, it may be necessary to increase resolution of the pattern including the interpolating pixels.
Japanese Patent Publication No. 4330164 (hereinafter referred to as “Patent Document 1”) discloses an interpolation technology.
However, in the interpolation technology disclosed in Patent Document 1, since the resolution of the analogous pattern utilized for the interpolation is the same as the resolution of the image, the pattern utilized for the interpolation may need to be highly analogous to the pattern including a pixel subject to interpolation in order to carry out accurate interpolation.
Meanwhile, in carrying out the interpolation on the halftone image utilizing the pattern matching, although dots of the halftone region have the same intensity, the dots of the halftone region may form different shapes according to positions of the image. This inconsistent shape may result from inconsistent sampling positions while sampling pixels of the image due to the resolution of the image failing to match an integral multiple of the lines per inch of the halftone dots. That is, even if the analogous pattern is searched for near the pixel subject to interpolation of a general halftone image, the analogous pattern having sufficiently matched pixels may not necessarily be found. The halftone dots form a two-dimensional pattern that has periodicity in a horizontal direction and a vertical direction. Accordingly, an optimal pattern for interpolation may not be found from the analogous patterns unless the pattern has phases matched with those of the pattern including the pixel subject to interpolation in both directions. If such a pattern is searched for in a wider range (area), the analogous pattern having the high matching level with that including the pixel subject to interpolation may be found. However, the more distance there is from the pixel subject to interpolation, the more likely the pattern may be affected by the content-dependent color or luminance. Consequently, the matching level may be degraded on the contrary.